J. Limnol., 63(1): 33-43, 2004
Carbon assimilation and phytoplankton growth rates across the trophic
spectrum: an application of the chlorophyll labelling technique
Giuseppe MORABITO*, Waleed HAMZA1) and Delio RUGGIU
CNR Istituto per lo Studio degli Ecosistemi, Largo V. Tonolli 50, 28922 Verbania Pallanza, Italy
1)
Biology Department, Faculty of Science United Arab Emirates University, P.O. Box 17551-Al-Ain, UAE
*e-mail corresponding author: [email protected]
ABSTRACT
The chlorophyll labelling technique has been acknowledged to be a useful method for measuring phytoplankton growth rates
while avoiding some of the problems involved in calculating growth rates derived from the 14C fixation rates. The results presented
here are of experiments comparing phytoplankton growth rates during the summer season in three subalpine Italian lakes: Lago
Maggiore, the second largest lake in Italy, and two smaller lakes, Lake Mergozzo and Lake Varese, both included in the Lago
Maggiore drainage basin. The three lakes have different morphometric, physico-chemical and biological features. The first goal was
to compare two different methods of estimating phytoplankton growth rates starting from 14C assimilation. The second goal of our
experiments was to test the hypothesis that growth rates can be quite different across the trophic spectrum, due to the ecophysiological and morphological features of the phytoplankton assemblages. In particular, algal cell size should decrease from eutrophic to
oligotrophic systems and growth rates should follow the opposite trend, as they are inversely scaled to the cell size. Two basic conclusions can be drawn. The first is that, in spite of some drawbacks still affecting the use of the chlorophyll labelling technique, this
appears to be one of the most promising methods for estimating the growth rates of phytoplankton in situ. The second conclusion is
that this method, coupled with information on some algal morphological parameters, can provide useful indications about the functional properties of phytoplankton assemblages living in diverse lacustrine environments.
Key words: primary productivity, chlorophyll labelling, phytoplankton growth rates, species assemblage structure
1. INTRODUCTION
The metabolism of a lake system is closely related to
ways of nutrient replenishment and the amount of nutrient supply, which largely determine its trophic status.
Although the uptake capacity of phytoplankton may exceed nutrient requirements for growth even in oligotrophic systems, growth rates show a wide range of amplitude. They usually mirror the ecophysiological features of the phytoplankton assemblage (Reynolds 1997),
whose composition is, in turn, variable across the trophic spectrum.
In particular, the physiology of the algal cell is
strictly dependent on its size: cell size is a key morphometric parameter, because many metabolic processes are scaled with the size of the cells (Reynolds
1997). In terms of photosynthetic efficiency, for instance, it is known that an increase in cell radius decreases the average specific absorption coefficient of
chlorophyll a (Raven & Kübler 2002), making smaller
algae more efficient. In terms of ecosystem functioning,
the accepted theory (see also Harris 1986, 1994) postulates that, when nutrients are scarce, phytoplankton is
dominated by small algae with high nutrient affinity,
and that the entire plankton community is driven by a
tight coupling between grazing and nutrient regeneration. On the other hand, when the nutrient load increases, the phytoplankton community will change in
composition towards species with a different metabolism, i.e. lower nutrient affinity, higher nutrient demands and larger cell volumes.
Therefore, estimation of phytoplankton growth rate,
coupled with analysis of phytoplankton assemblage
structure, can provide a number of useful indications on
the functioning of an aquatic ecosystem, because the
growth rate is related to the carbon pathways within the
food web, to the utilisation of nutrients and to the export
of biomass to other trophic levels. However, despite the
need for growth rate measurements, there have been few
field studies providing these data, mostly because the
measure of growth rate in situ involves serious methodological difficulties.
We used different approaches to evaluate phytoplankton growth rate. A common procedure for obtaining the growth rate is to divide the photosynthetic 14C
uptake by the carbon biomass (Steeman-Nielsen 1952).
However, a distinction between net and gross production from rates of 14C uptake cannot be achieved with
greater precision than 50%, as has been thoroughly reviewed by Peterson (1980) and Williams (1993). The
variability of the carbon-to-chlorophyll-a ratio can add a
further source of bias (Riemann et al. 1989). However,
Redalje & Laws (1981) suggested a direct method for
measuring growth rates of natural phytoplankton populations, which allows simultaneous measurements of the
growth rate and of the carbon specific biomass without
the need to know the carbon-to-chlorophyll-a ratio. The
34
G. Morabito et al.
Fig. 1. Lakes Maggiore (A) Mergozzo (B) and Varese (C) shown as shaded areas in a satellite image (source: European Space
Agency).
Tab. 1. Main morphometric parameters of the three lakes.
Altitude (m a.s.l.)
Drainage basin area (km2)
Volume (m3 106)
Area (km2)
Max depth (m)
Mean depth (m)
Turn over time (y)
L. Maggiore
L. Mergozzo
L. Varese
194
37500
6599
212
370
177.5
4.1
194
10.4
0.083
1.82
73
45.4
6
238
1.5
0.16
14.95
26
10.7
1.8
method is based on determination of the incorporation
rate of 14C into chlorophyll-a, and takes advantage of
the fact that chlorophyll is a stable end product in carbon assimilation and that apparently little carbon is respired from the synthesised chlorophyll-a (Hein & Riemann 1995).
In this study, we used the chlorophyll labelling technique to evaluate the carbon assimilation of summer
phytoplankton assemblages in three North Italian lakes:
the oligotrophic Lake Mergozzo, the oligo-mesotrophic
Lago Maggiore and the eutrophic Lake Varese. The
main goals of the research were i) to compare two different methods of estimating phytoplankton growth
rates starting from 14C assimilation, and ii) to evaluate
the validity of chlorophyll labelling as a direct method
of estimating algal growth rates in different trophic conditions. In discussing the results, we will take into account the variability of the morphometric features of
phytoplankton cells across the trophic spectrum.
2. THE LAKES
We chose as experimental sites three North Italian
lakes (Fig. 1) located in the western area of the subalpine lake district. The smaller two, lakes Varese and
Mergozzo, are included in the catchment area of Lago
Maggiore. Table 1 lists the main morphometric features
of the three basins, and table 2 shows their basic chemistry on the dates of the individual experiments.
These three lakes were chosen to compare the results
in lacustrine environments characterised by different
trophic states, morphology and morphometry and, presumably, by different phytoplankton assemblages. We
made two experiments per lake during summer, starting
on June 22 and finishing on July 7, with the sampling
periodicity reported in table 2. A short description of the
lakes’ history and a summary of earlier studies follows:
I. Lago Maggiore, the second largest Italian subalpine lake (Fig. 1A) is oligotrophic by nature, as testified
Carbon assimilation and phytoplankton growth rates
35
Tab. 2. Basic chemistry at the sampling dates. Data are integrated values in the euphotic zone.
N.D. = not detectable.
L. Mergozzo
L. Varese
L. Maggiore
Jun, 23
Jun, 29
Jun, 30
Jul, 05
Jun, 22
Jul, 07
pH
N-NO3
(µg l-1)
N-NH4
(µg l-1)
P-PO4
(µg l-1)
TP
(µg l-1)
Si
(mg l-1)
7.22
7.21
8.71
8.76
7.96
7.91
630
623
40
25
715
706
65
52
119
160
N.D.
N.D.
3
4
7
7
2
3
9
10
28
34
7
10
1.14
1.03
0.67
0.83
0.5
0.8
by early limnological studies (Monti 1929; Baldi 1949;
Vollenweider 1965) and by the analysis of the sedimentary pigments (Guilizzoni et al. 1983; Marchetto et
al. 2000). The eutrophication process started in the
1960s and the lake reached a trophic state close to eutrophy in the late 1970s (∼30 µg l-1; Mosello & Ruggiu
1985). Since that time, the P loads have been gradually
reduced. As a result, the values of TP at winter mixing
gradually decreased, to values around 10 µg l-1 in the
last few years (Calderoni et al. 1997). Many papers
document the slow reversal of the trophic state of Lago
Maggiore. Some major biological changes have occurred since 1987-88 (Manca et al. 1992; Ruggiu 1993),
and notable changes have also been recorded in the
structure of the phytoplankton assemblages with
oligotrophication (Ruggiu et al. 1998). Among the most
important of these is the remarkable decrease of the average cell size due to increased importance of the
smaller sized phytoplankters.
II. Lake Mergozzo (Fig. 1B) is a small lake that was
separated from Lago Maggiore by the alluvial deposits
of the River Toce five-six centuries ago. From the late
sixties the lake began to undergo a process of cultural
eutrophication, with the occurrence of Planktothrix
blooms and hypolimnetic oxygen depletion being recorded in the period 1969-1970 (Ruggiu & Saraceni
1972). As the result of a sewage diversion scheme, the
in-lake phosphorus started to decrease from the late
1980s, following a trend similar to that recorded in Lago
Maggiore. The present TP concentrations at winter
mixing are close to 4-5 µg l-1. Phytoplankton studies in
Lake Mergozzo are few and mainly cover the period of
maximum eutrophication (1970-1980; see Zutshi 1976);
in fact, information on the evolution of phytoplankton
biota in the last two decades is virtually absent.
III. Lake Varese (Fig. 1C) has a tendency to high
production because of its morphometric features and the
calcareous nature of its basin. The lake was already eutrophic at the beginning of last century (Guilizzoni et al.
1983), and the eutrophication process accelerated
greatly in the 1950s. Many studies testified to the worsening of the water quality and its effects on the biotic
communities (Bonomi 1962, 1966; Tonolli & Bonomi
1965). In the late 1960s work began on the construction
of a ring collector pipe designed to deviate the effluents
to a treatment plant. However, it was not until 1987 that
the plant was fully operational. Phytoplankton studies in
Lake Varese were carried out only occasionally. The
first detailed analysis dates back to 1979 (Ruggiu et al.
1981). The authors recorded high values of productivity
(550 g C m-2 as average annual productivity) and nutrient concentration (more than 400 µg l-1 TP at spring
overturn) and summer blooms of Planktothrix rubescens
and Microcystis aeruginosa. A further comprehensive
study, carried out about ten years later (Mosello et al.
1991), showed that, despite the effluent diversion, the
eutrophication process was not showing any sign of reversion, and indicated that a considerable release of nutrients from the bottom sediments was affecting the
lake. Some unpublished data collected during the last
decade on hydrochemistry and phytoplankton also indicate that the trophic conditions had not improved significantly.
3. METHODS
We carried out two experiments in summer (June–
July) in each of the three lakes, with an interval of one
to two weeks (Tab. 2). Usually we took five samples to
measure primary productivity, at depths corresponding to
specific surface PAR attenuation (100, 50, 25, 10 and 1%).
Underwater PAR was measured with a Li-Cor radiometer (Li-250, coupled with an underwater quantum
cosine sensor Li-192 SB).
Duplicate glass bottles (about 300 ml), inoculated
with NaH14CO3 (5 µCi), were suspended in situ for
about 4 hours around noon to measure productivity and
chlorophyll labelling. A blank radioactive sample was
prepared by inoculation of the same amount of
NaH14CO3 into a dark bottle, which was processed,
without incubation, like the in situ suspended bottles.
After incubation, a 30 ml subsample was filtered
through 0.2 µm Nucleopore filters to measure the POC
labelling, and the remaining sample (about 250 ml) was
filtered on GF/C glass fibre filters to estimate chlorophyll-a concentration and chlorophyll specific labelling.
After 90% acetone extraction, chlorophyll concentrations were determined spectrophotometrically. The
acetone was then evaporated under vacuum, the concentrated pigment extract (about 2 ml) poured into a liquid
scintillation vial and the activity of the sample determined.
Chlorophyll specific activity and growth rates were
finally calculated by the method of Redalje & Laws
(1981) and Redalje (1993). This method assumes that,
after a sufficiently long incubation, the specific activity
36
G. Morabito et al.
Fig. 2. Vertical profiles of carbon assimilation (bars) and PAR (lines) on the experimental dates in Lago Maggiore: black bars and
squares = Jun, 22 (Kd = 0.28); white bars and circles = Jul, 07 (Kd = 0.35).
of the chlorophyll-a carbon will become equal to that of
the total phytoplankton carbon pool. The parameter derived is a ratio of the mass of C in the isolated chlorophyll-a to the 14C activity contained in the isolated chl-a.
Phytoplankton determinations were carried out on
subsamples preserved in acetic Lugol's solution; algal
cells (including ultraplankton cells of about 1-2 µm diameter) were counted on a Zeiss Axiovert 10 microscope, following Lund et al. (1958), until 400 cells for
the most important species were counted. The phytoplankton countings were always performed on integrated samples from the euphotic zone. Because the
colonial species were counted as number of colonies
and not as number of cells, it was not possible to obtain
a direct measure of the phytoplankton biovolume.
Therefore, phytoplankton assemblage biovolume was
estimated from chlorophyll values, using the linear regression equation reported below, derived from a five
year set of data from Lago Maggiore (Morabito, unpublished data):
BV = 9.655 + 340.92×Chl-a
(n = 103; r = 0.818: p <0.001)
where Chl-a and BV are chlorophyll-a concentration
and phytoplankton biovolume, both estimated as integrated values in the euphotic zone. The application of
this regression to Lake Varese data might underestimate
the phytoplankton biovolume, as in this lake the algae
are on average larger than in Lago Maggiore. However,
we have to take account of the fact that the regression
line was drawn using data collected in different years
and seasons, so that the risk outlined above will probably be reduced.
Because the specific algal volumes were not calculated by direct measurement during this study, an average surface area-to-volume ratio of the whole phytoplankton assemblage was estimated for each experiment, using the ratios found in literature for the most
widespread species (Reynolds 1984 and references
therein), weighted for the density of the dominant species in our lakes. For each experiment we included in
the estimate most of the species present (contributing to
more than 95% of the total biomass).
The chemical parameters were determined on integrated water samples collected in the euphotic zone.
The analyses were carried out in the chemical laboratory
of the Istituto per lo Studio degli Ecosistemi (Pallanza),
following the methods reported in Mosello & Ruggiu
(1985).
4. RESULTS
4.1. Basic chemistry on the sampling dates
Basic chemical features showed an insignificant
variability between the two experiments in all three
lakes. So, at least from the chemical point of view, the
two experiments can be viewed as replicates. The values
of pH and of the main algal nutrients at the experimental
sites and periods are listed in table 2. Lakes Maggiore
and Mergozzo both showed similar concentrations of
nitrate nitrogen, and of inorganic and total phosphorus,
but we measured a higher silica concentration in Lake
Mergozzo in both samplings. In Lake Varese nitrate nitrogen was much lower than in the other two lakes,
whereas ammonium and TP values almost four times
higher were recorded. Silica concentration in Lake
Varese was intermediate among the three lakes.
4.2. Phytoplankton productivity and PAR attenuation
4.2.1. Lago Maggiore
We carried out two experiments in Lago Maggiore,
on June 22 and on July 7. In both experiments the euphotic zone extended below 10 m depth, reaching 14 m
in June and 12 m in July (Fig. 2). The increase of incident light just below the surface and the distortion of the
profile are probably due to the windy weather and sur-
Carbon assimilation and phytoplankton growth rates
37
Fig. 3. Chlorophyll specific rates versus light in lakes Maggiore (upper panels), Mergozzo (middle panels) and Varese (lower
panels).
face waves occurring at the time of measurement. In the
first experiment the vertical profile of carbon assimilation showed a subsurface maximum at 3 m depth, corresponding to 50% of surface PAR, whereas in July the
maximum assimilation was recorded at 5.5 m, corresponding to 25% of surface PAR. A higher chlorophyll
concentration in July (4.65 µg l-1, against 2.25 µg l-1 in
June, as weighted average in the euphotic zone) can explain the different amount of 14C assimilated.
The profile of the chlorophyll specific rates (photosynthetic rates) versus light shows a slight surface inhibition in both experiments (Fig. 3).
4.2.2. Lake Mergozzo
The experiments in Lake Mergozzo were both carried out in June (23 and 29). The PAR attenuation was
again low in this lake, where the euphotic depth reached
15 m in the first experiment and 18 m in the second
(Fig. 4). During the second experiment, because of
cloud cover, the amount of incident PAR was only
about 1/5 of that recorded during the first experiment.
This finding could account for the difference between
the two vertical profiles of carbon assimilation. In the
second experiment we observed a surface maximum,
probably due to a reduced inhibitory effect of the low
incident PAR, and a gradual decline towards the bottom
of the photic zone. In contrast, the profile recorded on
June 23 displayed an assimilation peak at 5 m (25% of
surface PAR). Average chlorophyll-a concentration in
the euphotic zone was similar in both experiments (3.31
and 3.42 µg l-1).
The photosynthetic rates vs light profiles of Lake
Mergozzo (Fig. 3) indicate a slight surface inhibition, as
already recorded in Lago Maggiore.
4.2.3. Lake Varese
The vertical profiles of PAR extinction and carbon
assimilation (Fig. 5) reflected the high trophic level of
Lake Varese. In both experiments we recorded a euphotic zone not deeper than 3.5 m, and a steep PAR attenuation, with 90% of surface light absorbed within the
first two meters of the water column. Productivity profiles were typical of eutrophic lakes, with a single, very
high peak located close to the surface. A dissimilar
amount of underwater PAR was perhaps responsible for
the quite different carbon assimilation rates recorded at
the peak value in the two experiments. This result suggests a light limitation in the water column of Lake
Varese. A deep chlorophyll peak was recorded on July 7
(68.7 µg l-1 at 3.5 m, lower limit of the euphotic zone).
Despite the shape of the vertical assimilation profile, the
photosynthetic rates (Fig. 3) indicate that the algae are
38
G. Morabito et al.
Fig. 4. Vertical profiles of carbon assimilation (bars) and PAR (lines) on the experimental dates in Lake Mergozzo: black bars and
squares = Jun, 23 (Kd = 0.30); white bars and circles = Jun, 29 (Kd = 0.25).
Fig. 5. Vertical profiles of carbon assimilation (bars) and PAR (lines) on the experimental dates in Lake Varese: black bars and
squares = Jun, 30 (Kd = 1.47); white bars and circles = Jul, 05 (Kd = 1.31).
well adapted to high light intensities: we did not observe
any light inhibition in either experiment.
4.3. Phytoplankton assemblages
The three lakes showed quite different phytoplankton assemblages (Tab. 3), but there were no significant
modifications in any lake from the first to the second
experiment, probably because of the short time intervals. This finding supports the hypothesis that differences between both productivity and growth rates could
be due to the variability of the light climate, rather than
to changes in the water chemistry or the phytoplankton
assemblages.
In Lago Maggiore, the dominant phytoplankton species were Cyclotella comensis, Tabellaria flocculosa,
Fragilaria crotonensis, Chrysochromulina parva, Uroglena americana and Ochromonas sp. In the second experiment a significant increase of Cyclotella comensis
and Chrysochromulina parva was observed, whereas
Uroglena americana and Ochromonas sp. declined.
Asterionella formosa and Sphaerocystis schroeteri
dominated the assemblage of Lake Mergozzo in both
experiments, with a slight decline from the first to the
second experiment. Chrysochromulina parva was also
important in this lake.
Uroglena americana and Ceratium hirundinella
were the dominant species in Lake Varese on the sampling dates. Dinobryon sociale and Rhodomonas minuta
reached quite high densities, with Chrysochromulina
parva and Chlamydomonas sp. showing some degree of
importance. All the species showed a decline between
the two experimental dates.
4.4. Growth rates
According to Redalje & Laws (1981), the most important constraint, when using the chlorophyll labelling
Carbon assimilation and phytoplankton growth rates
39
Tab. 3. Phytoplankton assemblages at the experimental dates in the three lakes. Data as cells or colonies ml-1. SA/V
Published = data from Reynolds (1984); SA/V Measured = directly estimated in 2003 from Lago Maggiore
phytoplankton.
SA/V
Published Measured
Cyanoprokaryota
Merismopedia punctata (col.)
Microcystis sp. (col.)
Anabaena solitaria (fil.)
Planktothrix rubescens (fil.)
Microcystis viridis (col.)
Microcystis aeruginosa (col.)
Aphanizomenon flos-aquae (fil.)
Bacillariophyceae
Asterionella formosa
Cyclotella comensis
Tabellaria flocculosa
Fragilaria crotonensis
0.03
1.03
0.52
0.07
0.43
1.62
0.50
1.16
0.5
0.71
1.5
0.11
0.70
1.11
1.30
1.47
0.76
0.76
1.5
0.030
0.43
2.2
0.047
Cryptophyta
Rhodomonas minuta
Cryptomonas erosa
1.5
0.35
2.1
0.38
0.219
0.32
0.26
Chlorophyta
Ankyra judayi
Chlamydomonas sp.
Monoraphidium spp.
Mougeotia sp.
Oocystis lacustris
Scenedesmus quadricauda
Sphaerocystis schroeteri
Staurastrum paradoxum
24
9
3
6
2242
Chryso-/Haptophyceae
Mallomonas tonsurata
Mallomonas sp.
Chrysochromulina parva
Dinobryon sertularia
Dinobryon sociale
Dinobryon divergens
Ochromonas sp.
Uroglena americana
Dinophyceae
Ceratium hirundinella
Gymnodinium spp.
L. Mergozzo
Jun, 26
Jun, 29
0.6
30
162
3
324
15
45
0.45
0.9
0.13
0.46
0.24
technique, is that a significant amount of labelled 14C
should become part of the chlorophyll molecule during
the incubation time. This requirement was satisfied by
our experiments, as we found a significant correlation
(R2 = 0.709; p <0.000001; n = 59) between the dpm
values of labelled chlorophyll and labelled total algal
biomass, measured as POC after filtration. Moreover,
the regression line of the two variables (dpm14Chl =
0.975dpmPO14C + 45.58) showed a slope very close to 1
and an intercept close to zero, indicating that most of
the 14C has been incorporated into chlorophyll.
The growth rates calculated from labelled chlorophyll in each experiment are shown in figure 6: in general, the highest values were recorded in Lago
Maggiore, followed by Lake Mergozzo and Lake
Varese in that order. We observed a clear relationship
with water depth in all three lakes, with the highest
growth recorded in the upper part of the water column
and the lowest at the limit of the euphotic zone. In Lake
21
27
18
9
48
24
9
21
L. Maggiore
Jun, 22
Jul, 07
9
6
120
147
171
534
72
114
211
2676
1347
30
60
21
213
453
672
25
97
2001
981
213
535
101
156
12
471
33
48
93
36
36
1548
1014
15
45
1.04
1
3
5
L. Varese
Jun, 30
Jul, 05
15
144
18
126
6
1491
2
24
981
36
12
15
3
6
48
39
29
Mergozzo there was a gradual decline of growth from
the surface towards the bottom of the illuminated layer;
in Lago Maggiore a less distinct pattern emerged, although a deeper growth peak seemed to prevail; in Lake
Varese the depth of 25% of subsurface PAR seems to be
optimal for growth. In this lake, both higher and lower
light intensities could be limiting or even damaging, as
observed in the second experiment, when we measured
negative growth just below the surface, because the
amount of radioactivity incorporated by the algae was
lower than the activity measured in the blank bottle.
Phytoplankton growth rates can also be calculated
from productivity values, as ratios between daily productivity and euphotic biomass expressed as carbon
(equivalent to a biomass turn-over rate; see Tilzer
1984). Our phytoplankton biovolumes were converted
to carbon using a biomass to carbon conversion factor
estimated by D. Ruggiu years ago for Lake Mergozzo
(algal carbon equal to about 13% of wet biomass; Sara-
40
G. Morabito et al.
Fig. 6. Comparison of the growth rates measured at different depths (shown as % of incident light) in the three lakes.
ceni et al. 1978). The growth rates estimated in this way
were always lower than the rates calculated from chlorophyll labelling; also, the differences among lakes were
less evident (Fig. 7).
Finally, the growth rates at each depth were compared with the average surface area-to-volume ratio
(SA/V) of the phytoplankton assemblages present in the
euphotic zone of the three lakes at the time of the experiments (Tab. 3): as shown in figure 8, a positive linear relationship exists between the two variables (r =
0.59; p = 0.001; n = 30). We are aware that a more
precise calculation of the SA/V ratios, i.e. using direct
phytoplankton measurements from each depth, would
have been better; however, our findings confirm the
general assertion that smaller algae have higher growth
rates (Reynolds 1997). In any case, we checked the
SA/V ratios published by Reynolds (1984) against the
ratios calculated in 2003 for most of the species found
in Lago Maggiore (unpublished data), finding a very
good correspondence (see Tab. 3).
5. DISCUSSION AND CONCLUSIONS
One of the most critical points of the chlorophyll labelling method is the time required for the specific activity of the chlorophyll-a carbon and total algal carbon
to become equal. Many studies (see e.g. Riemann et al.
1993 and references therein) demonstrated that the C
specific activity of different intracellular compartments
may vary markedly over the diel cycle and does not
follow the same temporal pattern for each constituent.
Riemann et al. (1993) observed that, in cells grown under a 12:12 L/D cycle, during an incubation lasting only
a few hours, the POC specific activities usually exceeded the Chl-a specific activities, although the dis-
crepancy was not always very large. The authors attributed this discrepancy to the fact that POC had a higher
specific growth rate than chlorophyll. However, this
pattern probably characterises cells growing under a
constant 12:12 L/D cycle. In this case the specific
growth rate of POC during the light period must be very
high, to compensate for the respiratory losses of carbon
during the dark period, when the inorganic C precursors
are used for the synthesis of chlorophyll. The case of algae grown in situ during early summer, when the light
period is longer than the dark one, might be different,
because the need to compensate for respiratory losses
during the night is minimised by the length of the light
period available for photosynthesis. This pattern could
explain the good relationship between POC and chlorophyll labelling shown by our data, even during short
term incubations.
Most of the differences in phytoplankton growth
rates recorded among the three basins could be due to
the variable species composition, with different morphological characteristics of the individual species. Of
these characteristics, the relationship between phytoplankton cell size and growth rate has often been investigated, and there seems to be general agreement about a
negative correlation between growth rate and cell size
(see Riegman et al. 1993 and references therein). Moreover, it has often been demonstrated that many physiological properties show an allometric relationship with
algal cell size (see the review by Capblancq & Catalan
1994). As a consequence of these relationships, many
environmental features, such as light availability, pattern of nutrient supply or grazing pressure, can constrain
the composition of a phytoplankton community towards
the dominance of large or small sized species. The re-
Carbon assimilation and phytoplankton growth rates
41
Fig. 7. Comparison of the growth rates estimated from the 14C assimilation (turnover rates) and from chlorophyll labelling.
Fig. 8. Relationship between growth rates and mean surface area-to-volume ratio in the three lakes Maggiore (squares), Mergozzo
(circles) and Varese (triangles).
sulting community dynamics will ultimately be a complex function of the spectrum of cell size and environmental variability. Because many environmental variables affecting cell physiology are the same as those
which define the trophic state of a lake, a link between
trophic spectrum and phytoplankton cell morphology
can be commonly found and has frequently been described in the literature. Some papers (Watson & Kalff
1981; Wehr 1993) demonstrated that smaller cells, with
their high surface area-to-volume ratio, generally have
high nutrient uptake affinities and thus an advantage in
lakes with a low nutrient load. For instance, picoplankton is more abundant with decreasing trophic status, because these small algae can have a competitive advantage in nutrient exploitation (Callieri & Stockner 2000).
However, differences found in the average community
cell size do not necessarily imply a change in the growth
rate: Hein & Riemann (1995) recorded an increase of
the community mean cell volume in enclosures enriched
with nitrogen and phosphorus, with no change in the
community growth rate. On the other hand, species of
similar size can exhibit different trophic preferences and
growth rates (e.g. Spijkerman & Coesel 1998). As a
general pattern, the decrease of the average community
cell size following a decrease in trophic status is well
documented (Edmonson 1991; Ruggiu et al. 1998;
Willén 2001) and, together with a general rearrangement of the whole planktonic food web, seems to indicate a change in the pathways of carbon transfer across it.
When these findings are kept in mind, our experimental results confirm that the chlorophyll labelling
technique is a promising method for measuring phytoplankton growth rates across the trophic spectrum: the
observed correlation between growth rates and average
42
surface area-to-volume ratio (Fig. 8) is in agreement
with the general relationship linking phytoplankton cell
size, growth rate and trophic level.
The two different methods used to estimate phytoplankton growth rates (from 14C incorporation into POC
and into chlorophyll) lead to quite different results: in
particular, the calculation of the biomass turn-over rate
from 14C incorporation into POC seems to be less precise in distinguishing the variable activity of different
phytoplankton assemblages. It is quite common to observe large discrepancies between rates of photosynthetic carbon fixation and phytoplankton growth rates
(Harris 1986). As pointed out by Capblancq & Catalan
(1994), the main reason for these discrepancies lies in
the difficulty of accurately estimating the carbon content of phytoplankton biomass. This estimate can give
unreliable values, due to the use of conversion factors
and to assuming the weight of cell carbon to be a constant fraction of cell weight.
A comparison of our results with the growth rate
values reported in literature is of little interest, because
the few papers reporting measures of phytoplankton
growth rates deal mostly with cultures, with marine
phytoplankton and, in general, with species different
from those we found, grown in different environmental
conditions. Moreover, as far as we know, no other papers have been published comparing the values of
growth rates obtained with chlorophyll labelling with
those calculated from biomass turn-over rates. On the
other hand, the validity of the labelling technique to
measure phytoplankton growth rates under variable ecophysiological conditions has been shown in various in
situ experimental studies (Redalje & Laws 1981;
Welschmeyer & Lorenzen 1984; Goericke &
Welschmeyer 1993, 1998; Hein & Riemann 1995; Goericke 1998), of which the study by Neale et al. (2001)
is the only one carried out in freshwaters.
Although some basic assumptions of the chlorophyll
labelling technique are still to be verified (Riemann et
al. 1993), the use of this method has yielded realistic results. An overestimation of the specific activity of chlorophyll-a has been reported (Jespersen et al. 1992), ascribable to contamination by non-pigmented lipid compounds (Goericke 1992; Pinckney et al. 1996) or caused
by isotope dilution (Riemann et al. 1993). Although a
possible overestimation of growth rates cannot be excluded, because of the low radiochemical purity of chlorophyll-a extract, we believe that, since the technical error is similar in all experiments, it will not influence the
validity of the following basic conclusions of our research:
- in spite of some drawbacks still affecting the use of
the chlorophyll labelling technique, such as the
choice of the correct incubation time and the estimation of the chlorophyll specific activity, the
method is probably one of the most promising for
estimating the growth rates of phytoplankton in situ;
G. Morabito et al.
-
this method, coupled with information on some algal
morphological parameters, such as surface area
and/or volume, can provide useful indications about
the functional properties of phytoplankton assemblages living in diverse lacustrine environments.
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